The Polish Foreign Ministry said it had summoned a Belarusian diplomat for a “break-in” at the country’s border.
Poland says that “unidentified uniformed persons with long guns” entered its territory on Monday night.
The Foreign Ministry called the incident a “provocation” and said it had summoned the Belarusian Chargé d’Affaires Alexander Chesnovsky.
The incident occurs because Poland is facing significant migratory pressure on its eastern border with Belarus.
Stanislaw Zaryn, a spokesman for the Polish Security Service, said the incident was “the most dangerous and serious to date”.
“After meeting a Polish patrol, they reloaded their weapons and then set out for Belarus,” said Zaryn. The incident happened around 300 meters from the Belarusian border, he added.
Poland’s Deputy Foreign Minister Piotr Wawrzyk called for an explanation, saying Belarusian officials risk escalating tensions.
“The actions taken by the Belarusian authorities in recent weeks are showing increasingly clear signs of deliberate escalation,” he said in a statement.
“Poland considers such actions unacceptable and will not tolerate them … Poland is determined to defend its borders and the external borders of the European Union,” he added.
In recent months, thousands of migrants from Syria, Iraq, Afghanistan and Africa have entered Poland and other EU member states, many on Belarusian tourist visas.
The European Union has accused Minsk of deliberately promoting illegal migration in retaliation against EU sanctions in order to destabilize the bloc.
A number of migrants and refugees remain stranded between the two borders in increasingly difficult autumn weather conditions.
At least ten people have been killed in the region so far, including an Iraqi migrant who was found on the Polish side of the border on Friday.
Poland has sent thousands of soldiers to the border and declared a state of emergency in the region to combat the surge in migrant arrivals.